The present invention relates to a spindle structure for a machine tool, and more particularly to a spindle structure for a machine tool, having a rotatable tubular shaft and a spindle axially slidably disposed in and rotatable in unison with the tubular shaft, the spindle structure having means for preventing abnormal wear from occurring on sliding surfaces of the tubular shaft and the spindle
There are widely used machine tools having a spindle with a cutting tool attached and a rotatable tubular shaft coupled to a rotative drive source, the spindle being axially slidably disposed in and rotatable in unison with the tubular shaft.
The machine tools suffer from a problem in that fretting corrosion is produced on sliding surfaces of the spindle and tubular shaft. It has been customary to avoid such fretting corrosion by carburizing and quenching or nitriding an inner peripheral surface of the tubular shaft which is slidably held against the spindle, or by placing a bushing made of cast bronze or a bushing with a thermally sprayed layer of powder of Mo, in the tubular shaft by shrink fit or cooling fit and thermally treating the spindle by carburizing and quenching or nitriding, or by selecting a suitable combination of materials of the spindle and the tubular shaft. Another method has been to supply oil under pressure to the sliding surfaces of the spindle and tubular shaft for preventing fretting corrosion.
While the spindle is rotating at a low or medium speed, any fretting corrosion can be prevented from occurring by employing the conventional heat treatment process or a combination of suitable materials. However, inasmuch as the rotational speed of machine tool spindles goes higher in recent years, the problem of fretting corrosion cannot fully be prevented by the measures heretofore available.
With oil supplied under pressure to the siding surfaces of the spindle and tubular shaft, the amount of heat produced by shearing of the oil becomes greater as the spindle rotates at a higher speed. Therefore, a device for cooling the oil is required to be larger in capacity, resulting in a higher cost.